I contacted Greg Bender at the weekend, and he was kind enough to reply with some insightful suggestions.

Apart from the obvious points like making sure the pivots and linkages are moving freely, he also responded to a question about whether there could be anything inside the box causing the issue, and he responded with this below:-

External to
the transmission, I would make sure that all of the linkages and pivot bushings
are in good shape, clean, lubricated, etc. If that still does not solve the
problem, then I would look internal to the transmission. Specifically, I would
look at the shift drum and examine how it rotates on its shaft and against the
thrust washers at either end. Any of those pieces can become corroded and make
for something that doesn’t want to rotate.

I'm not keen on getting into the box myself, as Dave Richardson says that they are not easy to re-assemble, and I think you need a few special tools too, so I don't see much point in ending up with a pile of parts on the bench. I guess Nigel at NBS would be the candidate for that job.

I want to try it out on the road for a longer run and see how it fares. At the moment I have to nudge the lever back up slightly, so it's ready for the next up-change.

One thing I did do was to mod the eccentric adjuster screw. As you know, it is only slotted on the end for means of adjustment, which makes it nigh on impossible to adjust when everything is back together. So I drilled and tapped the outer end of it and loctite'd an M6 bolt into it, so that it can be adjusted from the side rather than having to get on to the end of it. I also marked one of the flats on the bolt in line with the highers point of the eccentric on the other end for reference when adjusting it. I find a pair of ratchet spanners positioned from the side just above the UJ gaitor provide the best access.

After all that, Greg mentioned that he has never found that adjusting screw makes very much difference to the lever action in his experience.

Hi Pete, this is a bit strange as all was apparently well before you changed the spring. Was it shifting OK before? Or did you change the spring because of poor shifting?When I rebuilt my box I shimmed the shifting drum which made the lever return just a tad reluctant. Not wishing to strip it all again I rode it for a while and it freed up nicely. I think it just polished the surface of the shim washer which was plated.

The pedal was sticking intermittently and the return action was weak, which prompted the spring swap.

The doesn't do a lot of miles, as I generally tour on the 850 T and I've been using the Convert for most of the local miles this year, so I'm wondering whether it could be lack of miles that is causing the pedal to stick a bit. But I could be clutching (no pun intended) at straws?

You have obviously carried out surgery on your box and survived to tell the tale?

Just a thought, or another straw to grasp. Is the gear lever shaft binding in the tunnel through which it passes? Is it clean and greased? And does it have end float? Ah! Greg's already asked you that.

I think we should have an emoticon on here of the little yellow chap reaching for a straw.

After making no progress at all with this gearbox problem. I took the box over to Nigel at NBS before Christmas for some investigative surgery. After collecting it today, he found that the shaft that selector drum runs on was worn, and causing the drum to bind. Also the selector shaft that the return spring is fitted to was one tooth out of alignment on the gear teeth it meshes with. That must have been me when I changed the spring. In my feeble defence, it's not easy to see when the box is still in the bike.

I will get it back in the bike over the next couple of weeks and hopefully enjoy the improvement.

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